1. Field of the Invention
The present invention relates, generally, to pressure chambers. More particularly, the invention relates to Stirling engines with a dual shell pressure chamber.
2. Background Information
The maximum Stirling engine efficiency is related to the Carnot efficiency which is governed by the ratio of maximum working fluid temperature relative to the minimum fluid temperature. Improvements in technologies which increase the margin between the two temperature extremes is beneficial in terms of total cycle efficiency. The lower working fluid temperature is typically governed by the surrounding air or water temperature; which is used as a cooling source. The main area of improvements result from an increase in the maximum working temperature. The maximum temperature is governed by the materials which are used for typical Stirling engines. The materials, typically high strength Stainless Steel alloys, are exposed to both high temperature and high pressure. The high pressure is due to the Stirling engines requirement of obtaining useful power output for a given engine size. Stirling engines can operate between 50 to 200 atmospheres internal pressure for high performance engines.
Since Stirling engines are closed cycle engines, heat must travel through the container materials to get into the working fluid. These materials typically are made as thin as possible to maximize the heat transfer rates. The combination of high pressures and temperatures has limited Stirling engine maximum temperatures to around 800° C. Ceramic materials have been investigated as a technique to allow higher temperatures, however their brittleness and high cost have made them difficult to implement.
U.S. Pat. No. 5,611,201, to Houtman, shows an advanced Stirling engine based on Stainless Steel technology. This engine has the high temperature components exposed to the large pressure differential which limits the maximum temperature to the 800° C. range. U.S. Pat. No. 5,388,410, to Momose et al., shows a series of tubes, labeled part number 22 a through d, exposed to the high temperatures and pressures. The maximum temperature is limited by the combined effects of the temperature and pressure on the heating tubes. U.S. Pat. No. 5,383,334 to Kaminiishizono et al, again shows heater tubes, labeled part number 18, which are exposed to the large temperature and pressure differentials. U.S. Pat. No. 5,433,078, to Shin, also shows the heater tubes, labeled part number 1, exposed to the large temperature and pressure differentials. U.S. Pat. No. 5,555,729, to Momose et al., uses a flattened tube geometry for the heater tubes, labeled part number 15, but is still exposed to the large temperature and pressure differential. The flat sides of the tube add additional stresses to the tubing walls. U.S. Pat. No. 5,074,114, to Meijer et al., also shows the heater pipes exposed to high temperatures and pressures.
The Stirling engine disclosed in the inventor's U.S. Pat. No. 6,041,598 overcomes the limitations and shortcomings of the above prior art by providing a dual shell pressure chamber. An inner shell surrounds the heat transfer tubing and the regenerator. The portion surrounding the heat transfer tubing contains a thermally conductive liquid metal to facilitate heat transfer from a heat source to the heat transfer tubing and also to transmit external pressure to the heat transfer tubing. An outer shell that acts as a pressure vessel surrounds the inner shell and contains a thermally insulating liquid between the inner and outer shells. Pressure of the working fluid as it flows through the regenerator is transmitted through the inner shell to the insulating liquid and back across the inner shell to the liquid metal surrounding the heat transfer tubing. This system tends to balance the pressure across the heat transfer tubing and the inner shell, thereby allowing the engine to operate with the working fluid at a high pressure to generate significant power while keeping the wall of the heat transfer tubing thin to facilitate heat transfer.
The preferred material for the insulating liquid is a salt or glass such as Boron Anhydride or a mixture of Boron Anhydride and Bismuth Oxide. Those materials are fairly viscous when liquid, but still allow significant convection currents. A filler material such as ceramic fiber or similar material is placed in the liquid salt region to minimize convective currents. While this can work very well to transmit and balance the pressure across the inner shell and across the heat transfer tubing, combining the filler material and the liquid salt and installing it between the shells in a manner that does not produce voids can be difficult. Also, before the salt melts it does not transmit pressure. Therefore, significant preheating must be done to thoroughly melt the salt before the engine can be run with significant pressure in the working fluid.
The present invention improves on the dual shell pressure chamber and overcomes the difficulties in using the insulating liquid between the shells by using gas instead of a liquid.